1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and, more particularly, to an LCD device and a fabrication method thereof. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for improving a residual image level of an LCD device.
2. Description of the Related Art
In general, the LCD device is a display device in which data signals corresponding to image information is supplied to a matrix of pixels. The data signals control optical transmittance of the pixels so as to display images. The LCD device includes a liquid crystal display panel in which the pixels are arranged in a matrix and a driving part for driving the pixels. The liquid crystal display panel includes an array substrate on which a thin film transistor (TFT) array is formed and a color filter substrate on which color filters are formed. The TFT array substrate and the color filter array substrate are attached to each other with a uniform cell gap maintained therebetween with a liquid crystal layer positioned within the cell gap. The array substrate and the color filter substrate are attached by a seal pattern formed along an outer edge of a pixel part.
Alignment films are formed on surfaces of the array substrate and the color filter substrate that face each other. The alignment films are rubbed to make liquid crystals be arranged in a predetermined direction. A polarization film and a retardation film are provided on each outer surface of the TFT array substrate and the color filter substrate. By selectively constructing a plurality of components, a liquid crystal display panel can have high luminance and good contrast characteristics by changing the direction and/or refracting the proceeding light.
FIG. 1 is a plan view of a related art LCD device. As shown in FIG. 1, the LCD device includes a pixel part 35 having pixels arranged in a matrix for displaying an image, a gate pad part 31 electrically connected with the gate lines 16 of the pixel part 35, and a data pad part 32 electrically connected with the data lines 17 of the pixel part 35. The gate pad part 31 and the data pad part 32 are formed at an edge portion of the array substrate 10, which is not overlapped by a color filter substrate 5. The gate pad part 31 supplies a scan signal from the gate driving unit (not shown) to the gate lines 16 of the pixel part 35, and the data pad part 32 supplies image information from the data driving unit (not shown) to the data lines 17 of the pixel part 35. The data lines 17 and the gate lines 16 are arranged to cross each other on the array substrate 10 to define pixel regions. A thin film transistor (not shown) and pixel electrodes (not shown) are provided in the pixel regions defined by the data lines 17 and the gate lines.
Although not shown in FIG. 1, color filters for each of the pixels are separated by a black matrix. Further, a common electrode, which is a counter electrode of the pixel electrode formed on the array substrate 10, is formed on the color filter substrate 5. A certain cell gap is maintained between the color filter substrate 10 and the array substrate 5 by spacers (not shown), and the color filter substrate 10 and the array substrate 5 are attached by a seal pattern 50 formed along an outer edge of the pixel part 35. In such an LCD device shown in FIG. 1, in general, parasitic capacitance (Cgd) is generated where the gate electrode and the source electrode overlap and/or where the gate electrode and the drain electrode of the TFT overlap. This parasitic capacitance causes a change in a voltage by an amount equaling ΔVp, which is called a level shift voltage or a kickback voltage.
The capacity of liquid crystal to maintain proper orientation degrade when a DC voltage is applied across liquid crystal in one direction for a long time. Accordingly, when liquid crystal is driven, the polarity of an applied voltage must be periodically changed. Due to the kickback voltage, a DC component from the parasitic component remains that causes bad effects, such as flickering, residual screen images and non-uniform screen brightness. Each of the TFTs in the pixel regions have a different kickback voltage depending on their positions in the panel. Thus, because the amount of the DC component remaining in the panel is different across the panel, a non-uniform residual image is generated.
FIG. 2 is a graph showing variation values of common voltages according to each position on the panel as simulated by a computer. As shown in FIG. 2, points indicated by a diamond shape and square shape show variation values of common voltages according to each position on the panel from a computer simulation while points indicated by a triangular shape show an average value of the simulation results. The X axis in the graph of FIG. 2 indicates a relative position on the panel and the Y axis indicates the size of each common voltage. In other words, the left side of the X axis refers to the left side on the panel based on the center of the panel while the right side of X axis refers to the right side on the panel. As shown in FIG. 2, the size of each common voltage is not consistent across the panel. More particularly, the common voltages differ depending on the position in the panel along the horizontal direction of the panel. Since the amount of the accumulated DC component is different depending on position in the panel, a non-uniform residual image (ghost) is created across the entire panel, which cannot be resolved by the existing method of uniformly compensating the common voltages across the panel.